How to fix my shower

I recently purchased my house and found that my shower walls are not really secured into place when you lean on them or put pressure on them they have a lot of flexibility.

Where the 3 corner panels meet at the 2 joints is where it is a problem. Once you put pressure on the corners the caulking overtime starts coming off .

Is there anything I can do to secure these corners better so there is less give so I am able to caulk them with no further problems.

I do not really want to take all of the inserts out and redo the whole shower if I do not have to because they sheet rocked around the corners of the shower so it would be quite a bit of work to do it all correctly and add extra supports behind the shower walls. I have thought about putting shims in the corners while I add extra caulking in the corners then let them dry and seal the seams like you would normally but im not sure it would work.